Hantavirus pulmonary syndrome (HPS) is a frequently fatal zoonosis that is endemic in the New World. The virulence of HPS appears to be dependent upon hantaviral genetics. The results of recent studies suggest that the virulence of the New World hantaviruses in Syrian golden hamsters, as in humans, is dependent upon viral genetics. Thus, the hamster model of HPS provides a unique opportunity to elucidate the viral determinants of virulence. The long-term objectives of the proposed research are to 1) elucidate the genetic elements of the hantaviral genome that individually or collectively determine the virulence of the New World hantaviruses, 2) extend our knowledge of the pathogenesis of HPS, 3) identify antiviral compounds for therapy of HPS, and 4) develop vaccines against HPS. Aim #1 is to develop a plasmid-based genetic system that will enable scientists to map the determinants of virulence to specific elements of the hantaviral genome. Components of the genetic system will be used in future studies to rapidly and economically screen for compounds that can prevent hantaviral infection or disease. Aim #2 is to rigorously compare and contrast the pathogenicity and virulence of 6 different but genetically very closely related New World hantaviruses. Aim #3 is to determine the genetic sequences of the complete genomes of these 6 viruses. The genetic sequence database will be used in combination with the experimental animal data from Aim #2 to identify elements of the hantaviral genome that may be causally associated with virulence. Aim #4 is to assess the contribution of the large, medium, and small segments of the hantaviral genome to the virulence of HPS-like disease in the hamster. The plasmid-based genetic system from Aim #1 will be used to generate reassortant viruses from Maporal virus (MAPV), which is highly virulent in hamsters, and a hantavirus that is not virulent in hamsters and, importantly, that is genetically closely related to MAPV. The results of the work in Aim #4 are expected to narrow the focus of future studies to identify the viral determinants of virulence. Aim #5 is to investigate the effect of the small, medium, and large genomic segments of MAPV on the evolution of HPS-like disease in hamsters. A series of time-course studies will be done to assess the association between 1) genomic segment, 2) kinetics of viral infection and host response to infection, and 3) virulence. Elements of the viral genome that align with virulence may be appropriate targets for sequence-based antiviral therapy or vaccine development. The plasmid-based genetic system could be used in future studies to develop vaccines to prevent infection or disease and to (in the face of clinical disease) mitigate the severity of HPS.